Casing for electronic components

ABSTRACT

A copper plastic laminate is provided having a high bond strength. The copper is selected from the group consisting of copper and copper alloys and has on its surface a uniform glassy-like, substantially pore-free coating of copper phosphate. A plastic encapsulating material containing a mold release agent is adhesively bonded to the copper.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Division of U.S. application Ser. No. 443,793,pending filed Nov. 22, 1982, by Sheldon H. Butt et al. for AdhesionPrimers For Encapsulating Epoxies.

While the invention is subject to a wide range of applications, it isespecially suited for encapsulating electrical leadframes withintegrated circuits attached thereto and will be particularly describedin that connection.

In the electronics industry, one of the basic types of semiconductorpackages used for integrated circuits is the plastic molded package. Thepackage may include a leadframe, having an electronic device attachedthereto, molded into an encapsulating plastic. This type of plasticpackage often has reliability problems created by failures in theplastic to metal bond between the encapsulating plastic and theleadframe. The bond failure provides an avenue through which moistureand other atmospheric contaminants can reach the electric device andcause corrosion problems. Such failures are more fully explained anddocumented in an article entitled "Factors Governing AluminumInterconnection Corrosion in Plastic Encapsulated MicroelectronicDevices" by Neighbour and White published in Microelectronics andReliability, by Pergamon Press in Great Britain, Volume 16, 1977, pages161-164.

It is known in the prior art to produce a laminate of copper or copperalloys and plastic film. A laminating adhesive is applied between themetal and plastic film to bond them together. A high bond strengthresults between the copper or copper alloys and the plastic film due toa phosphate coating provided on the metal. Examples of this aredisclosed in U.S. Pat. Nos. 3,677,828, 3,716,427, 3,728,177, 3,728,178,3,764,399, 3,833,433, 3,837,929, 3,853,691, 3,940,303, 3,941,627,3,941,628, and 3,944,449. However, the plastic film laminate disclosedin these patents do not contain mold release agents as set out in thepresent invention. Since the adhesive is commonly required only betweenthe film and the metal, there is no reason to use a mold release agent.Further, if a mold release agent were used, it would only decrease thebond strength. The absence of mold release agents is significantlydifferent from the present invention where the mold release agents aredefinitely required to prevent an encapsulating epoxy from sticking to amold surface. Unfortunately, the mold release agent also hinders theencapsulant from sticking to an encapsulated metal strip such as aleadframe. The resulting bond between the encapsulant and the leadframeis often deficient and creates an avenue for contaminating anencapsulated electronic element attached to the leadframe.

It is a problem underlying the present invention to provide anencapsulated package for an electrical or electronic component which ishighly resistant to atmospheric contamination.

It is an advantage of the present invention to provide a casing for anelectrical or electronic component which obviates one or more of thelimitations and disadvantages of the described prior arrangements.

It is a further advantage of the present invention to provide a hermeticcasing for an electrical or electronic component which is substantiallyresistant to the diffusion of contaminants.

It is a yet further advantage of the present invention to provide acasing for an electrical or electronic component which is relativelyinexpensive to manufacture.

Accordingly, there has been provided a casing adapted for containing anelectrical or electronic component and a method of producing the casing.The casing includes a copper or copper alloy leadframe. An adhesionprimer comprising a uniform, glassy and substantially pore-freephosphate coating is applied to the leadframe. Also, an encapsulatingmaterial containing a mold release agent is bonded to the copperleadframe to form a hermetic casing. In addition, the encapsulatingepoxy may be adhered to copper or copper alloy material having anadhesion primer thereon whenever a strong bond is required.

The invention and further developments of the invention are nowelucidated by means of the preferred embodiments shown in the drawings.

FIG. 1 is a graph of a lap shear strength test of uncoated copperalloys;

FIG. 2 is a graph of a lap shear strength test of coated copper alloys;

FIG. 3 is a graph of a durability test for coated and uncoated copperalloys.

The present invention is primarily concerned with forming a strong bondbetween copper or copper alloy and an encapsulating epoxy or plastic.This is particularly important in semiconductor leadframe applicationswhere the leadframe and semiconductor attached thereto are encapsulatedby an encapsulating epoxy. Semiconductor devices are particularlysensitive to degradation and failure from exposure to moisture and/orpenetration of atmospheric pollutants into the package through a pathbetween the encapsulating molding compound and the leadframe. Althoughthis is the primary application described herein, it is within the scopeof the present invention to apply the treatment described hereinwherever copper or copper alloys are molded into the general class ofplastics or epoxies which contain mold release agents. The mold releaseagent also known as an abherent allows the hardened epoxy to releasefrom the encapsulating mold and minimize redressing the mold after eachuse. Just as the mold release agent prevents the plastic or elastomerfrom sticking to the mold wall, it also reduces the strength of the bondbetween the metal or alloy leadframe and the plastic or elastomer.

The metal or alloy which is used in this invention is preferably copperor copper alloy. Also, this metal or alloy may be in any form such assheets, strip, or foil.

The metal or alloy is coated with a uniform, glassy-like, substantiallypore-free phosphate coating. This coating may be in the range ofthickness from about 20 to about 100 Angstrom units. It may be appliedusing various techniques as set forth in the patents enumerated above.For example, the coating may be obtained by applying a phosphoric acidsolution containing from about 3.5 grams per liter up to the solubilitylimit of sodium dichromate (Na₂ Cr₂ O₇ 2H₂ O) or potassium dichromate(K₂ Cr₂ O₇) or mixtures thereof to the copper alloy material. Normally,the application of the aforementioned solution is by immersion of asheet or strip of material into a bath of the above-mentioned acidsolution.

Another treatment which is thought to be advantageous in the presentinvention is disclosed in U.S. Pat. No. 4,264,379.

The treatment may be effected with an aqueous solution containing a lowto moderate concentration of the phosphonic acid component orcomponents, preferably ranging from about 0.1 to about 30 volume percentfor liquid acids or corresponding weight percent limits for solidphosphonic acids, preferably in the range of about 0.1 to 40 percent byweight.

The treating solution also preferably includes a low to moderateconcentration, such as about 0.1 to about 15.0 percent by weight,preferably 0.2 to 5.0 percent by weight of oxidizing agent, such assodium or other alkali chromate or dichromate, or nitric acid (100percent) at a concentration of about 0.05 to about 10.0 volume percent,preferably about 0.05 to about 2.0 percent by volume HNO₃. Other knownoxidizing agents of similar activity may be used at a comparable diluteor moderate concentration effective for the purpose, but generally withavoidance of such vigorous oxidizing conditions as might causesubstantial decomposition of the phosphonic acid.

Following the aforementioned immersion step, the copper alloy strip isrinsed and dried. The rinsing is normally carried out in running wateralthough a spray rinse may also be readily employed. Drying may beaccomplished by an air blast, rinsing in an alcohol solution such asmethanol and allowing to dry, or merely by exposure to the atmosphere.

Following rinsing and drying, the treated surface of the copper sheet orstrip is prepared for the attachment of semiconductors and theattachment of lead wires from the semiconductors to the coated leadframeor strip. In manufacturing the leadframe with a semiconductor deviceattached thereto, it may be desirable to silverplate a spot on theleadframe to enhance the connection of the wires from the semiconductordevice to the leadframe. Further, the ends of the leadframe may beelectrosolder plated to enhance the connection between the leadframe andanother element to which it may be attached by a soldering operation. Inany case, the glassy-like phosphate coating of the leadframe, inaccordance with the present invention, is preferably accomplished afterthe addition of the silverplated spot and/or electrosolder plate becausethe coating does not substantially adhere to either the silver or thesolder. Conversely, the silverplating and/or electrosolder plate may beapplied after the phosphate coating has been applied because the coatingdoes not prevent the adherence of the plating.

The semiconductor device is attached to the plated leadframe in anyconventional manner. The leadframe may then be placed within a mold andencapsulated in any conventional manner with an epoxide resin formulatedfor integrated circuit encapsulation, preferably containing both moldrelease agents and inert fillers which are suitable for the inventiondescribed herein. The encapsulating epoxies are generally in the classof thermosetting plastic or polymers. They preferably contain moldrelease agents, such as low melting temperature organics, which allowrelease of the hardened epoxy from the mold and minimize mold redressingbetween uses. These mold release agents or abherents may includematerials such as silicones, stearates and fatty acids. Examples ofother abherents are described in the Encyclopedia of ChemicalTechnology, 3rd Edition, by Kirk-Othmer, published by John Wiley & Sons,Volume 1, pages 1-9. The encapsulating epoxies may also contain inertfillers to provide dimensional and temperature stability. These fillersmay be discrete particles such as glass fibers or silica.

Epoxy encapsulants, containing mold release agents, which are suitablefor the present invention are Morton Polyset 410B manufactured by MortonChemical Company and Plaskon 3100 and 3200 manufactured by PlaskonElectronic Materials, Inc.

FIGS. 1-3 illustrate the experimental results exhibited during lap shearstrength testing of samples. The lap shear strength test measures thebond strength in terms of shear strength. It is performed by overlappingtwo strips of the desired material and bonding them together with anepoxy. Then, the strips are pulled apart from each other exerting ashear stress at the bond.

Referring to FIG. 1, there is shown samples of uncoated CDA copper stripsubjected to the lap test described above. The encapsulating epoxy usedin this test was Morton Polyset 410B. FIG. 2 illustrates samples of CDAcopper strips coated with glassy-like phosphate and bonded together withMorton Polyset 410B. Comparison of FIGS. 1 and 2 indicates that themedian bond strength is nearly double for the coated copper alloysamples as compared to the clean samples of the same alloy.

FIG. 3 illustrates the experimental results of a durability test. Thedurability test measures the effect of the moisture on the bondstrength. The data illustrated resulted from bonding coated and uncoatedcopper alloy CDA 194 strips with an encapsulating epoxy, Morton Polyset410B. The epoxy was cured for 24 hours at 165° C. Then, the bondedstrips were placed in boiling water for different amounts of time.Afterwards, they were pulled apart using the aforementioned lap strengthtest. The slope through the median of the bond strength ranges of thecoated and uncoated or clean copper 194 indicates that the coated alloyhad a larger bond strength. This is an example of a specific cure cycle.The same type of effects can be expected with any equivalent cure cycle.Even in an uncured condition, specific bond improvement of about 285 PSIfor coated copper alloy vs. 200 PSI for uncoated copper alloy was found.

The durability tests indicate the increased bond strength of coatedstrip in accordance with the present invention, as compared to uncoatedstrip when both were subjected to similar conditions of heat andmoisture. The test is a strong indicator of the hermeticity of a packagecomprising a semiconductor device attached to a leadframe andencapsulated with epoxy containing mold release agents as well as inertfillers. The hermeticity has been substantiated by vacuum testingplastic metal adhesion bonds with and without the adhesion primers. Inthe absence of an adhesion primer, no vacuum is obtained. However, withcoated samples, vacuum measuring in the range of 10⁻⁶ and 10⁻⁷ Torr wasachieved.

The coating of the present invention is thought to provide an additionaladvantage in preventing bond failure in the shear mode due todifferences in thermal expansion between the metal and the epoxy. Theexpansion coefficient of epoxy may be different from the expansioncoefficient of the copper or copper alloy. For example, the coefficientof thermal expansion of copper is 10×10⁻⁶ in/in/°C. and the coefficientof linear expansion of a typical encapsulating epoxy is 27×10⁻⁶in/in/°C. The difference in coefficient of expansions creates a highshear stress condition when the composite is placed in a cyclictemperature environment. However, the adhesion primers of the presentinvention are thought to be slightly flexible and have been shown toadjust to some expansion. The improved bond strength achieved with thepresent invention should prevent bond failure over a wider sheardisplacement range, such as exists in the range of temperatures between-50° C. and 165° C. These temperatures present the extremes in whichthese devices are normally used. This may be particularly important inadverse environmental conditions where encapsulated semiconductordevices are used.

The patents and publications set forth in the specification are intendedto be incorporated by reference herein.

It is apparent that there has been provided in accordance with thisinvention a copper plastic composite and a method of forming thecomposite which fully satisfies the objects, means, and advantages setforth hereinabove. While the invention has been described in combinationwith the specific embodiments thereof, it is evident that manyalternatives, modifications, and variations will be apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended to embrace all such alternatives, modifications, andvariations as fall within the spirit and broad scope of the appendedclaims.

We claim:
 1. A casing adapted to contain an electrical or electroniccomponent, comprising:a copper or copper alloy leadframe; an adhesionprimer on the surface of said leadframe, said adhesion primer comprisinga substantially uniform glassy and substantially pore-free phosphatecoating; and a plastic material comprising an epoxide resin bonded tosaid leadframe to form said casing, said resin forming a strong adhesivebond to said leadframe, said epoxide resin containing a mold releaseagent.
 2. The casing as in claim 1 wherein said plastic materialcomprises an epoxide resin formulated as a thermosetting encapsulatingepoxy.
 3. The casing as in claim 2 wherein said plastic materialcomprising an epoxide resin further contains inert fillers to providedimensional stability to said casing.
 4. The casing as in claim 3wherein said mold release agent comprises low melting temperatureorganics.
 5. The casing as in claim 4 wherein said mold release agent isselected from the group consisting of silicones, stearates and fattyacids.